Morphology and classification of some Early Silurian monograptids (Graptoloidea) from the Cape Phillips Formation, Canadian Arctic Islands

Abstract

Exquisitely preserved early to mid-Llandovery monograptids have been chemically isolated from calcareous concretions sampled from the Cape Phillips Formation, central Canadian Arctic Archipelago. They provide morphological information critical in refining the definition of four of the earliest known monograptid genera, and in the elucidation of their phylogenetic dispersal through the Llandovery monograptid radiation. These primitive monograptid genera are defined primarily on the basis of derived thecal form. The specific, derived thecal and thecal apertural character states used in generic diagnoses are as follows: species of Atavograptus possess simple, geniculate to slightly sigmoidal thecae with a large dorsoventral overlap; species of Pribylograptus possess thecae bearing lateral apertural lappets that may occur with Atavograptus-style thecae proximally; species of Coronograptus possess flared thecal apertures at least part way along the rhabdosome; species of Lagarograptus have conspicuous, tongue-like, ventral apertural processes developed as ventral apertural margin outgrowths. Species of all of these genera may possess hoods that are genicular in origin, with the exception of those of Lagarograptus inexpeditus, which are dorsal wall structures. These earliest monograptid genera possess a pattern Mr astogeny, characterized by the outward, then upward growth of theca 1 from a sicular resorption porus (pattern Mr, different from younger monograptids that possess a primary porus developed during sicular astogeny (pattern Mp)). Through analysis of their defining morphological criteria in conjunction with sicular dimensions and thecal uniformity, each of these early monograptid genera is considered to be monophyletic, arising from the primitive Atavograptus stock. Atavograptus thecal morphology provided a simple template for the derivation of thecal form leading to the nearly simultaneous evolution of Pribylograptus, Coronograptus, and Lagarograptus in the lower Rhuddanian. Fourteen species belonging to four genera are described, including six new species.